Fmt provides printing to standard output (usually the console) and formatting of strings capabilities.
The basics of Print and Println
The main difference between fmt.Print and fmt.Println is that you only get a newline/carriage return by default with the second one. Other than that, you can make use of either for string formatting.
Firstly, we need to import the fmt module as follows:
package main
import "fmt" // <--- include fmt
func main() {
// main
}
Example of fmt.Print
fmt.Print("This is a Print test")
// "This is a Print test"
This will print to standard output without a newline at the end.
Example of fmt.Println
fmt.Println("This is a Println test")
// "This is a Println test\n <--- newline added
// "
Printing variables
const number = 37
fmt.Println("The number is", number)
// "The number is 37"
Notice how a space is automatically added between the strings above.
Printing complex structures
items := []int {10, 20, 30, 40, 50}
length, err := fmt.Println(items)
fmt.Println(<meta charset="utf-8">length, err)
// [10 20 30 40 50]
// 14 nil
The above prints out the list of item, followed by the length of that string, and nil as the error.
Introducing Printf and Sprintf
These are quite similar to the Print methods we looked at previously, but they offer more flexibility for formatting.
These functions make use of the formatting verbs, such as %v, %#v, %T and %t to name yet a few. You can get a full list from https://pkg.go.dev/fmt
.
How to print decimal values
fmt.Printf("%d\n", 20)
// 20
How to print hexadecimal values
fmt.Printf("%x\n", 123.45)
// 14
How to print a boolean value
fmt.Printf("%t\n", 2 > 1)
// true
How to print floating point numbers
fmt.Printf("%f\n", 123.45)
// 123.450000
How to print exponent numbers (scientific notation)
fmt.Printf("%e\n", 123.45)
// 1.234500+02
More advanced usages
fmt.Printf("%d, %d", 10, 20)
// 10 20
fmt.Printf("%[2]d %[1]d", 10, 20)
// 20 10
// decimal, octal and hexadecimal
fmt.Printf("%d %#[1]o %#[]x\n", 52)
52 064 0x34
Formatting by precision
Decimal precision
fmt.Printf("%.2f\n", 123.4567)
// 123.46
Print with width and precision
// print with 10 spaces
fmt.Printf("%10f\n", 123.4567)
// 123.456700
Print with padding and precision
fmt.Printf("%10.2f\n", 123.4567)
// " 123.46"
Print with + sign
fmt.Printf("%+10.2f\n", 123.4567)
// " +123.46"
Pad with 0s instead of spaces
fmt.Printf("%010.2f\n", 123.4567)
// 0000123.46
